My memory of that 1969 championship team is that they were old and Russell was hurt for a significant portion of the season. Were they saving themselves for the playoffs?
I don't know.
According to wikepedia:
However, in the 1968–69 season, Russell seemed to reach a breaking point. Shocked by the murder of Robert F. Kennedy, disillusioned by the Vietnam War, and weary from his increasingly stale (and later divorced) marriage to his wife Rose, he was convinced that the U.S. was a corrupt nation and that he was wasting his time playing something as superficial as basketball.[46] He was 15 pounds overweight, skipped mandatory NBA coach meetings and was generally lacking energy: after a New York Knicks game, he complained of intense pain and was diagnosed with acute exhaustion.[46] Russell pulled himself together and put up 9.9 points and 19.3 rebounds per game,[21] but the aging Celtics stumbled through the regular season. Their 48–34 record was the team's worst since 1955–56, and they entered the playoffs as only the fourth-seeded team in the East.
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Once they got into the playoffs, they were a different team. I still can't believe that they beat the Knicks that year let alone the Lakers.
That 1969 championship still remains my favorite moment in a lifetime of following sports.